Ischemia-reperfusion (I/R) injury is characterized by autologous inflammatory attack that is mediated by natural IgM antibody and complement. A single IgM clone produced by a murine peritoneal B1 cell hybridoma triggers reperfusion injury of skeletal muscle, intestine, and scald burns in rodent models. This natural IgM antibody binds exposed self-antigens on ischemic tissue and activates complement, leading to tissue injury. An analogous pathologic mechanism of I/R injury in humans has been proposed but is unproven. Preliminary data from our group demonstrate that adoptive transfer of human IgM into RAG2 -/-mice reconstitutes reperfusion injury of murine and human tissue. The goal of this proposal is to identify human IgM involved in ischemia-reperfusion injury. We hypothesize that human peritoneal B1 cells harbor one or several antibody determinants that can bind self antigen and initiate the inflammatory cascade leading to I/R injury. We will test this hypothesis through the following specific aims: (1) cloning of immunoglobulin molecules expressed by human peritoneal B1 cells using single cell RT-PCR;(2) expression and screening of human natural IgM for reconstitution of reperfusion injury in antibody deficient RAG2 -/- mice and of burn injury in SCID mice bearing human skin grafts. Identification of a human natural antibody involved in ischemia-reperfusion injury will provide potential therapeutic targets for down regulating I/R injury and thus improving outcomes in stroke, heart attacks, and surgery. Characterization of the human natural antibody repertoire would also be an important tool for understanding B1 cells and their role in host defense and autoimmunity.